In a photographic image consisting of silver, the ratio of the optical density of the image to the amount of image-constituting silver per unit area is generally called the covering power. The covering power is adopted as a measured of evaluating the optical efficiency of the image-constituting silver. The covering power of a silver halide photographic light-sensitive layer is, in general, increased by a decrease in the size of the silver halide grains. The grain size is generally represented by a diameter or a reduced diameter. The covering power is, in general, decreased by an increase in the size of the silver halide grains. On the other hand, an increase in the size silver halide grains can, in general, increase the sensitivity of a silver halide emulsion layer. Therefore, a silver halide emulsion having a large grain size is employed in highly sensitive photographic materials. Consequently, the highly sensitive photographic material requires a larger amount of silver per unit area in order to attain a prescribed image density. More specifically, a photographic material must contain a larger amount of silver halide per unit area in order to attain both high sensitivity and maximum image density. Accordingly, a large amount of silver is used in preparing conventional highly sensitive photographic materials.
An attempt to improve upon the covering power while maintaining high sensitivity is disclosed in British Pat. Nos. 1,048,057 and 1,039,471, and U.S. Pat. Nos. 3,043,697 and 3,446,618. These patents describe a technique of adding various kinds of polymers to highly sensitive coarse grain silver halide emulsions. However, the addition of such polymers has been found to be insufficient with respect to improvement in the covering power. Furthermore, the addition of these polymers lowers the stength of coated films.
When photographic materials constituted with coated films having low mechanical strength are processed using a general present-day automatic developing machine, a noticeable amount of the gelatin contained in the coated films dissolves out into the developing solution or the fixing solution. The dissolved gelatin comes to adhere to the conveying roller of the automatic developing machine. The thus adhered gelatin is then transferred onto photographic materials causing stains on photographic images. Therefore, such a technique is not considered to be suitable for the above-described purpose.
On the other hand, U.S. Pat. Nos. 2,996,382 and 3,178,282 describe another method for producing photographic images of high sensitivity, high contrast and high covering power. The method described in these patents comprises using a silver halide photographic material having surface latent image type silver halide coarse grains and silver halide fine grains having fogged nuclei inside thereof in the same layer or in adjacent layers separately. According to this method, it was believed that the surface latent image type silver halide coarse grains are at first developed, and the resulting development products attack the silver halide fine grains, which have fogged nuclei inside thereof, present in the neighborhood of the products to cause development of the fine grains. However, when such a photographic material as described above is processed with a developing solution having a conventional formula, it requires too long of a developing time to be of practical use. Accordingly, the above-described U.S. Patents disclose the addition of silver halide solvents, such as potassium thiocyanate, thiosulfates and so on, to conventional developing solutions. However, as described in each of the above referred to U.S. Patents, reduction of developing time is achieved by such a method, but graininess of image is deteriorated and fog is increased. This defect cannot be remedied by the addition of generally used anti-fogging agents (e.g., mercaptotetrazoles).